Method of forming gate insulating film for thin film transistors using plasma oxidation

ABSTRACT

In forming a thin film transistor, to form a film superior in quality to a film formed by a conventional CVD method and to form a film equal or superior in quality to a film formed by a thermal oxidation method at a temperature which does not affect a substrate. Plasma oxidation or plasma nitridation with a low electron temperature and a high electron density is performed to at least one of a glass substrate, a semiconductor film containing amorphous silicon formed into a predetermined pattern, a gate electrode and a wire pulled from the gate electrode, an insulating film to be a gate insulating film, and a protective film with a temperature of the glass substrate set at a temperature 100° C. or more lower than a strain point of the glass substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention disclosed in this specification relates to asemiconductor device having a thin film transistor such as formation ofa gate insulating film of a thin film transistor or a protective film ofa gate electrode.

2. Description of the Related Art

A thin film transistor is widely known as a switching element used in anactive matrix display device. In a manufacturing process of a thin filmtransistor, a CVD method or a thermal oxidation method hasconventionally been employed in general so as to form an insulatingfilm.

However, there has been a problem in that a silicon oxide film formed bya CVD method is inferior to a silicon oxide film obtained by a thermaloxidation method in film quality, such that the silicon oxide filmformed by a CVD method lacks in density, contains much impurities suchas carbon, and suffers a damage due to plasma (plasma damage).

On the contrary, in order to efficiently form a silicon oxide filmhaving a predetermined thickness with high quality by a thermaloxidation method, it is required to oxidize silicon in an oxygenatmosphere at a temperature of 800° C. or more. Thus, in the case ofemploying a thermal oxidation method in forming a gate insulating filmof a thin film transistor, a glass substrate typified by non-alkaliglass cannot be used and a quartz substrate which is more expensive thanthe glass substrate is forced to be used.

In addition, when thermal oxidation is performed to silicon having acorner portion, a thickness of a silicon oxide film formed over thecorner portion of the silicon becomes thinner in some cases, comparedwith a thickness of a silicon oxide film formed over a roughly planeportion of a top surface of the silicon. This is because oxidation issuppressed due to stress caused by a shape of the corner portion.

In the future, it is required to make a thinner gate insulating filmthan ever before in accordance with more miniaturization of a thin filmtransistor. For example, although a gate insulating film isconventionally formed with a thickness of 100 nm or more, it is requiredto be formed with a thickness of several tens of nm. However, in thecase of using a silicon oxide film formed by the above-describedconventional method as a gate insulating film, the thinner the thicknessthereof becomes, the more the amount of leakage current flowing betweena semiconductor film including a channel formation region and a gateelectrode via the thin silicon oxide film is increased. Further, in acase where a silicon oxide film formed as a gate insulating film doesnot have a uniform thickness and locally has a thin portion, there is apossibility of generating leakage current via the thin portion.

As a material for forming the gate insulating film, silicon oxynitride(denoted by SiO_(x)N_(y), note that x>y) is sometimes used instead ofsilicon oxide. However, heat treatment at a high temperature exceeding astrain point of a glass substrate is required to form the siliconoxynitride film by heat treatment in an atmosphere such as N₂O which iscapable of performing nitridation.

Recently, a method of forming a gate insulating film of a field effecttransistor for an LSI with a plasma treatment apparatus which is capableof performing plasma oxidation and plasma nitridation has been focused.For example, it is disclosed in Reference 1 that a silicon nitride filmto be a gate insulating film is formed over a semiconductor layer bydirectly reacting nitrogen activated by plasma excitation with siliconof the semiconductor layer (Reference 1: Japanese Patent Laid-Open No.2004-319952). However, according to Reference 1, disclosed are only anexample of using an SOI (Silicon On Insulator) substrate and a pointthat the semiconductor layer may be a bulk semiconductor substrate, andan attempt to form a gate insulating film of a thin film transistor withan apparatus capable of performing plasma oxidation and plasmanitridation is not disclosed.

SUMMARY OF THE INVENTION

It is an object of the present invention disclosed in this specificationto obtain an insulating film, in a manufacturing process of a thin filmtransistor, which is superior in quality to an insulating film formed bya film formation method of a conventional CVD method. It is anotherobject of the invention to obtain an insulating film having an equal orsuperior quality to an insulating film formed by heat treatment at ahigh temperature using a thermal oxidation method, at a temperaturewhich does not affect a glass substrate. It is a further object of theinvention to form a protective film (passivation film) over a gateelectrode of a thin film transistor by a similar method to the case ofthe above-described insulating film. This protective film (passivationfilm) is also called a barrier film. The insulating film mentioned abovehas to have a sufficient quality as a gate insulating film of a thinfilm transistor, and the protective film mentioned above has to have asufficient quality as a protective film formed in contact with a gateelectrode of a thin film transistor.

An apparatus capable of performing plasma oxidation and plasmanitridation is used in forming a gate insulating film of a thin filmtransistor or in forming a protective film of a gate electrode of a thinfilm transistor. In this apparatus, plasma is excited in a chamber usingmicrowaves, and an electron temperature of 1.5 eV or less (preferably1.0 eV or less) and an electron density of 1×10¹¹ cm⁻³ or more can beconcurrently achieved with no magnetic field over a treatment subjectsuch as a semiconductor film, an insulating film, or a gate electrode.In this specification, this apparatus is hereinafter called ahigh-density plasma treatment apparatus. Accordingly, since it becomespossible to generate plasma with high density at a low electrontemperature, plasma damages to a gate insulating film and a protectivefilm to be formed can be suppressed.

The plasma is an ionized gas in which approximately equal amounts ofelectrons having negative charge and ions having positive charge exist,and is electrically natural on the whole. Note that the number ofelectrons or the number of ions included per unit area of the plasma iscalled a plasma density, and the plasma density indicates an electrondensity in the invention disclosed in this specification. In addition,radicals which are electrically natural are generated in the plasma, andthe radicals affect a treatment subject which is subjected to plasmatreatment. Thus, plasma oxidation and plasma nitridation hereinafterdescribed in this specification is, in some cases, called radicaloxidation and radical nitridation, respectively.

One feature of the invention disclosed in this specification is amanufacturing method of a thin film transistor including the steps offorming a base insulating film over a glass substrate, forming apredetermined pattern of a semiconductor film containing amorphoussilicon over the base insulating film, forming an insulating film (gateinsulating film) by performing plasma oxidation or plasma nitridation tothe semiconductor film containing amorphous silicon having thepredetermined pattern with a condition where a temperature of the glasssubstrate is set at a temperature 100° C. or more lower than a strainpoint of the glass substrate, and forming a gate electrode and a wirepulled from the gate electrode over the insulating film. The plasmaoxidation or plasma nitridation mentioned above is performed above theglass substrate which is set away from a plasma generation region in anapparatus including a plasma treatment chamber in which an electrontemperature of 0.5 eV or more and 1.5 eV or less and an electron densityof 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or less are concurrently achievedwith no magnetic field. After performing the plasma oxidation, theplasma nitridation may be further performed, or after performing theplasma nitridation, the plasma oxidation may be further performed. Inaddition, the plasma nitridation may be performed to the glasssubstrate.

One feature of the invention disclosed in this specification is amanufacturing method of a thin film transistor including the steps offorming a base insulating film over a glass substrate, forming apredetermined pattern of a semiconductor film containing amorphoussilicon over the base insulating film, forming an insulating film overthe semiconductor film containing amorphous silicon having thepredetermined pattern, forming a gate electrode and a wire pulled fromthe gate electrode over the insulating film, and forming a protectivefilm by performing plasma oxidation or plasma nitridation to the gateelectrode and the wire with a condition where a temperature of the glasssubstrate is set at a temperature 100° C. or more lower than a strainpoint of the glass substrate. The plasma nitridation mentioned above isperformed above the glass substrate which is set away from a plasmageneration region in an apparatus including a plasma treatment chamberin which an electron temperature of 0.5 eV or more and 1.5 eV or lessand an electron density of 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or lessare concurrently achieved with no magnetic field. The plasma nitridationmay be performed to the glass substrate.

One feature of the invention disclosed in this specification is amanufacturing method of a thin film transistor including the steps offorming a base insulating film over a glass substrate, forming apredetermined pattern of a semiconductor film containing amorphoussilicon over the base insulating film, forming a gate insulating film byperforming plasma oxidation or plasma nitridation to the semiconductorfilm containing amorphous silicon having the predetermined pattern witha condition where a temperature of the glass substrate is set at atemperature 100° C. or more lower than a strain point of the glasssubstrate, forming a gate electrode and a wire pulled from the gateelectrode over the gate insulating film, and forming a protective filmby performing plasma oxidation or plasma nitridation to the gateelectrode and the wire with a temperature of the glass substrate set at100° C. or more lower than a strain point of the glass substrate. Theplasma oxidation or plasma nitridation mentioned above is performedabove the glass substrate which is set away from a plasma generationregion in an apparatus including a plasma treatment chamber in which anelectron temperature of 0.5 eV or more and 1.5 eV or less and anelectron density of 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or less areconcurrently achieved with no magnetic field. After performing theplasma oxidation, the plasma nitridation may be further performed, orafter performing the plasma nitridation, the plasma oxidation may befurther performed so as to form the gate insulating film. The plasmanitridation may be performed to the glass substrate.

One feature of the invention disclosed in this specification is amanufacturing method of a thin film transistor including the steps offorming a base insulating film over a glass substrate, forming apredetermined pattern of a semiconductor film containing amorphoussilicon over the base insulating film, forming an insulating film overthe semiconductor film containing amorphous silicon having thepredetermined pattern, performing plasma oxidation or plasma nitridationto the insulating film with a condition where a temperature of the glasssubstrate is set at a temperature 100° C. or more lower than a strainpoint of the glass substrate so as to form a gate insulating film, andforming a gate electrode and a wire pulled from the gate electrode overthe gate insulating film. The plasma oxidation or plasma nitridationmentioned above is performed above the glass substrate which is set awayfrom a plasma generation region in an apparatus including a plasmatreatment chamber in which an electron temperature of 0.5 eV or more and1.5 eV or less and an electron density of 1×10¹¹ cm⁻³ or more and 1×10¹³cm⁻³ or less are concurrently achieved with no magnetic field. A siliconoxide film containing nitrogen, a silicon oxide film, a silicon nitridefilm, or a silicon nitride film containing oxygen formed by a CVD methodor the like can be given as an example of the insulating film. Theplasma nitridation may be performed to the glass substrate.

One feature of the invention disclosed in this specification is amanufacturing method of a thin film transistor including the steps offorming a base insulating film over a glass substrate, forming apredetermined pattern of a semiconductor film containing amorphoussilicon over the base insulating film, forming an insulating film overthe semiconductor film containing amorphous silicon having thepredetermined pattern, performing plasma oxidation or plasma nitridationto the insulating film with a condition where a temperature of the glasssubstrate is set at a temperature 100° C. or more lower than a strainpoint of the glass substrate so as to form a gate insulating film,forming a gate electrode and a wire pulled from the gate electrode overthe gate insulating film, and forming a protective film by performingplasma oxidation or plasma nitridation to the gate electrode and thewire with a temperature of the glass substrate set at 100° C. or morelower than a strain point of the glass substrate. The plasma oxidationor plasma nitridation mentioned above is performed above the glasssubstrate which is set away from a plasma generation region in anapparatus including a plasma treatment chamber in which an electrontemperature of 0.5 eV or more and 1.5 eV or less and an electron densityof 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or less are concurrently achievedwith no magnetic field. A silicon oxide film containing nitrogen, asilicon oxide film, a silicon nitride film, or a silicon nitride filmcontaining oxygen formed by a CVD method or the like can be given as anexample of the insulating film. The plasma nitridation may be performedto the glass substrate.

The invention disclosed in this specification is not limited to atop-gate (planar) thin film transistor, and can also be applied to amanufacturing process of a bottom-gate thin film transistor.

In a case where the bottom-gate thin film transistor is manufactured, agate electrode and a wire pulled from the gate electrode can be formedwithout forming a base insulating film over a glass substrate. Then, aninsulating film is formed over the gate electrode, and a gate insulatingfilm is formed by performing plasma oxidation or plasma nitridation tothis insulating film with a condition where a temperature of the glasssubstrate is set at a temperature 100° C. or more lower than a strainpoint of the glass substrate. Over the gate insulating film, asemiconductor film containing amorphous silicon is formed, and thebottom-gate thin film transistor is thereafter completed by a knownmethod. The plasma oxidation or plasma nitridation mentioned above isperformed above the glass substrate which is set away from a plasmageneration region in an apparatus including a plasma treatment chamberin which an electron temperature of 0.5 eV or more and 1.5 eV or lessand an electron density of 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or lessare concurrently achieved with no magnetic field. A silicon oxide filmcontaining nitrogen, a silicon oxide film, a silicon nitride film, or asilicon nitride film containing oxygen formed by a CVD method or thelike can be given as an example of the insulating film. The plasmaoxidation or plasma nitridation may be performed to the gate electrodeand the wire pulled from the gate electrode.

The electron temperature of 0.5 eV or more and 1.5 eV or less and theelectron density of 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or less set ineach of the above-described methods are conditions for reducing plasmadamages and sufficiently performing either of plasma oxidation or plasmanitridation. In addition, a reason for setting the temperature of theglass substrate at a temperature 100° C. or more lower than the strainpoint of the glass substrate is because heat resistance of the glasssubstrate is considered. In the case of using a glass substrate having astrain point of 650° C. or more, the temperature 100° C. or more lowerthan the strain point is preferably 550° C. or lower Since a glasssubstrate using alkali glass or non-alkali glass has a strain point ofover 500° C., plasma oxidation or plasma nitridation can be performed ata temperature of 400° C. or lower which is a temperature 100° C. or morelower than the strain point of the glass substrate. In addition, thetemperature of the glass substrate is necessarily 200° C. or more, andis preferably 250° C. or more to perform plasma oxidation or plasmanitridation with the above-described high-density plasma treatmentapparatus.

Instead of the glass substrate, a heat-resistant plastic substrate canbe used. Thermoplastic polyimide (TPI) is one of the heat-resistantplastic. A temperature of the heat-resistant plastic substrate inperforming plasma oxidation or plasma nitridation is necessarily setequal to or lower than a glass transition point of the heat-resistantplastic substrate used and 200° C. or more. In the case of the inventiondisclosed in this specification, it is preferable to use aheat-resistant plastic having a glass transition point of 200° C. ormore and preferably 250° C. or more. In addition, a quartz substratehaving higher heat resistance than that of the glass substrate may beused.

By the plasma oxidation or the plasma nitridation, an oxide (oxide film)or nitride (nitride film) is formed on a surface of a semiconductor filmcontaining amorphous silicon, an insulating film, a protective film, ora glass substrate. An active matrix display device is manufactured byusing a thin film transistor including such an oxide (oxide film) ornitride (nitride film). In addition, an active matrix display device ismanufactured by using a thin film transistor including a semiconductorfilm containing amorphous silicon, an insulating film, or a protectivefilm subjected to the plasma oxidation or the plasma nitridation.

According to the invention disclosed in this specification, a dense andthin gate insulating film having a uniform thickness in which plasmadamages and generation of cracks are suppressed can be formed at atemperature which does not affect a glass substrate or a heat-resistantplastic substrate. A thin film transistor formed with such a gateinsulating film generates less leakage current via the gate insulatingfilm than ever before. In addition, a step of forming a gate insulatingfilm by a film formation method such as a CVD method can be omitted.

According to the invention disclosed in this specification, byperforming plasma oxidation or plasma nitridation to an insulating filmformed by a known film formation method such as a CVD method, forexample, a silicon oxide film containing nitrogen or a silicon nitridefilm, a dense gate insulating film can be formed at a temperature whichdoes not affect a glass substrate or a heat-resistant plastic substrate.A thin film transistor using such a gate insulating film generates lessleakage current via the gate insulating film than ever before. Further,a particle (dust) over a surface of a film (not limited to an insulatingfilm) formed by a film formation method such as a CVD method or asputtering method can be easily removed, and an impurity such as carbonin the film can be removed by plasma oxidation.

In addition, according to the invention disclosed in this specification,since a dense protective film having a uniform thickness in which plasmadamages are suppressed can be formed, heat resistance, corrosionresistance, and oxidation resistance of a gate electrode and a wirepulled from the gate electrode can be improved. In addition, a step offorming a protective film by a film formation method such as a CVDmethod can be omitted.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D are cross-sections showing a manufacturing process of athin film transistor corresponding to Embodiment Mode 1;

FIGS. 2A and 2B show an apparatus capable of performing plasma oxidationand plasma nitridation;

FIGS. 3A to 3D are cross-sections showing a manufacturing process of athin film transistor corresponding to Embodiment Mode 2;

FIGS. 4A to 4D are cross-sections showing a manufacturing process of athin film transistor corresponding to Embodiment Mode 3;

FIGS. 5A and 5B are cross-sections showing plasma treatment performed toan insulating film to which a dust is attached;

FIGS. 6A to 6D are cross-sections showing a manufacturing process of athin film transistor corresponding to Embodiment Mode 4;

FIGS. 7A to 7C are cross-sections showing a manufacturing process of athin film transistor corresponding to Embodiment Mode 5;

FIGS. 8A and 8B show an EL display device corresponding to Embodiment 1;

FIG. 9 shows a liquid crystal display device corresponding to Embodiment2;

FIGS. 10A to 10C show electronic devices corresponding to Embodiment 3;and

FIG. 11 shows changes in the average film thickness of a formed oxidefilm with respect to the plasma oxidation time corresponding toEmbodiment Mode 8.

DETAILED DESCRIPTION OF THE INVENTION

In embodiment modes described below, examples of performing plasmaoxidation or plasma nitridation in forming a thin film transistor willbe described. Each embodiment mode shall be appropriately implemented incombination with each other.

Embodiment Mode 1

As shown in FIG. 1A, a base insulating film 102 is formed over a glasssubstrate 101. Instead of using a glass substrate, a heat-resistantplastic substrate can be used. A structure formed of one layer or amultilayer can be employed for the base insulating film 102, and inEmbodiment Mode 1, a silicon nitride film containing oxygen and asilicon oxide film containing nitrogen (silicon oxynitride film)thereover are continuously formed by a CVD method or the like. Thepurpose of forming the base insulating film 102 is to prevent diffusionof impurities from the glass substrate 101 to a semiconductor film laterto be formed. Accordingly, since a silicon oxide film is not sufficientto achieve this purpose, a silicon nitride film or a silicon nitridefilm containing oxygen which can more effectively prevent diffusion ofimpurities than the silicon oxide film, needs to be formed. In addition,the silicon oxide film is superior to the silicon nitride film inattachment to silicon.

A semiconductor film 103 containing amorphous silicon is formed with apredetermined pattern, over the base insulating film 102. In thisembodiment mode, a semiconductor film containing amorphous silicon isformed by a CVD method or the like over an entire surface of the baseinsulating film 102, and is later formed into a predetermined pattern ina photolithography step. In the case where the semiconductor filmcontaining amorphous silicon is formed by a CVD method or the like, itmay be formed to contain germanium. In addition, the semiconductor filmcontaining amorphous silicon before being formed into the predeterminedpattern may be doped with impurities imparting p-type conductivity orimpurities imparting n-type conductivity. A thickness of thesemiconductor film 103 containing amorphous silicon has to be determinedwith considering the decreasing thereof in later performing plasmaoxidation or plasma nitridation.

An angle θ of a side surface of the semiconductor film 103 with respectto a surface of the glass substrate 101 or the base insulating film 102is in the range of 85° to 100°. Note that in forming the semiconductorfilm into a predetermined pattern, it may be formed into a tapered shapeso that the angle θ is in the range of 30° to 60°.

Plasma treatment is performed to the semiconductor film 103 with ahigh-density plasma treatment apparatus shown in FIGS. 2A and 2B. FIGS.2A and 2B show an example of the high-density plasma treatmentapparatus, and the invention is not limited to the structure shown intheses drawings.

The high-density plasma treatment apparatus includes, as shown in FIG.2A, at least a first plasma treatment chamber 201, a second plasmatreatment chamber 202, a load lock chamber 203, and a common chamber204. Plasma oxidation is performed in the first plasma treatment chamber201, and plasma nitridation is performed in the second plasma treatmentchamber 202. Each chamber of FIG. 2A is vacuum evacuated, and plasmaoxidation and plasma nitridation can be continuously performed withoutexposing to air. The high-density plasma treatment apparatus may furtherincludes at least one of a chamber for CVD, a chamber for sputtering,and a chamber for thermal annealing, in addition to the chambers shownin FIG. 2A, and thereby can continuously perform film formation andplasma treatment, or plasma treatment and thermal annealing withoutexposing to air.

A robot arm 205 is provided in the common chamber 204. In the load lockchamber 203, a cassette 206 in which a plurality of treatment substrates200 is stored is provided. One treatment substrate 200 stored in thecassette 206 can be transferred to the first plasma treatment chamber201 or the second plasma treatment chamber 202 through the commonchamber 204 by using the robot arm 205. In addition, the treatmentsubstrate 200 can be transferred from the first plasma treatment chamber201 to the second plasma treatment chamber 202 through the commonchamber 204 by using the robot arm 205, or can be reversely transferredfrom the second plasma treatment chamber 202 to the first plasmatreatment chamber 201 through the common chamber 204 as well.

FIG. 2B shows a common structure in the first plasma treatment chamber201 and the second plasma treatment chamber 202. A vacuum pump (notshown) capable of reducing the pressure to a predetermined value isconnected to the first plasma treatment chamber 201 and the secondplasma treatment chamber 202, and air is exhausted from an exhaust port210. In addition, a substrate holder 211 is provided in the first plasmatreatment chamber 201 and the second plasma treatment chamber 202, andthe treatment substrate 200 to be subjected to plasma oxidation orplasma nitridation is held on the substrate holder 211. This substrateholder 211 is also called a stage, and it is provided with a heater soas to heat the treatment substrate 200. A gas such as oxygen, nitrogen,hydrogen, a rare gas, or ammonia is introduced into the plasma treatmentchamber from a gas introduction opening as indicated by an arrow 212.Microwaves 213 for exciting plasma are introduced through a waveguide215 provided over an antenna 214. Plasma is generated in a shaded area217 just below a dielectric plate 216 with a pressure in the plasmatreatment chamber after introducing the above-mentioned gas of 5 Pa ormore and 500 Pa or less, and is supplied onto the treatment substrate200 which is provided away from the area 217. A shower plate 218 havinga plurality of holes may be provided as shown in FIG. 2B. Plasmaobtained in this plasma treatment chamber has an electron temperature of1.5 eV or less and an electron density of 1×10¹¹ cm⁻³ or more, in otherwords, achieves a low electron temperature and a high electron density,and has a plasma potential of 0 V or more and 5 V or less. Plasmaparameters about an electron temperature, an electron density, and aplasma potential can be measured by a known method, for example, a probemeasuring method such as a double probe method.

In this embodiment mode, oxygen, hydrogen, and argon are introduced intothe first plasma treatment chamber 201 with a flow ratio ofO₂:H₂:Ar=1:1:100, and plasma is generated using microwaves having afrequency of 2.45 GHz. Plasma oxidation can be performed withoutintroducing hydrogen; however, a flow ratio of hydrogen to oxygen(H₂/O₂) is preferably set in the range of 0 to 1.5. For example, anoxygen flow is set in the range of 0.1 sccm to 100 sccm, an argon flowis set in the range of 100 sccm to 5000 sccm, and, in the case ofintroducing hydrogen, a hydrogen flow is set in the range of 0.1 to 100sccm. Instead of argon, another rare gas may be introduced. A pressurein the first plasma treatment chamber 201 is set at an appropriate valuein the range of 5 Pa to 500 Pa. The glass substrate 101 is provided onthe substrate holder 211 of the first plasma treatment chamber 201, anda temperature of the heater provided under the substrate holder 211 iskept at 400° C. Then, plasma oxidation is performed to the semiconductorfilm 103 over the glass substrate 101. In this embodiment mode, as isapparently shown in FIG. 1B, a portion of the base insulating film 102,which is not covered by the semiconductor film 103, is also plasmaoxidized. Note that in the case where the base insulating film 102 ismade from an oxide, an oxide film is not formed over a surface of thebase insulating film 102 even when plasma oxidation is performed.

By the plasma oxidation described above, an oxide film 104 to be a gateinsulating film shown in FIG. 1B is formed with a thickness of 20 nm orless. In the oxide film 104, argon introduced into the first plasmatreatment chamber 201 is contained with a predetermined concentration,for example 1×10¹⁵ atoms/cm³ or more and 1×10¹⁶ atoms/cm³ or less. Whenthe oxide film 104 is formed too thin, tunneling current (leakagecurrent) may be generated. Accordingly, the thickness is set at 10 nm,for example. Since a corner portion of the semiconductor film 103becomes rounded when forming the oxide film 104, a thickness of theoxide film 104 formed over the corner portion does not become thinnerthan that of other portions. In addition, there is no possibility ofcausing a crack in the oxide film 104 over the corner portion. In thecase of using a heat-resistant plastic substrate instead of using theglass substrate 101, a temperature of the heater provided under thesubstrate holder 211 is, for example, kept at 250° C.

Since plasma over the semiconductor film 103 has an electron temperatureof 1.5 eV or less and an electron density of 1×10¹¹ cm⁻³ or more, plasmadamages to the oxide film 104 obtained by plasma oxidation aresuppressed. By using microwaves of 2.45 GHz so as to generate plasma, alow electron temperature and a high electron density can be more easilyrealized than in the case of using a frequency of 13.56 MHz. Inaddition, as long as a low electron temperature and a high electrondensity can be obtained, a method other the method using microwaves of2.45 GHz may be employed.

The oxide film 104 may be used as a gate insulating film; however, ifplasma nitridation is further performed to the oxide film 104 in thesecond plasma treatment chamber 202 to form into a silicon oxynitridefilm, the silicon oxynitride film can be used as a gate insulating film.As a gas introduced into the second plasma treatment chamber 202 inplasma nitridation, nitrogen and argon are used, and a temperature ofthe glass substrate is set at the same temperature as in the case of theabove-described plasma oxidation. Hydrogen may be further added to thenitrogen and argon, and another rare gas may be used instead of usingargon. Instead of nitrogen, a gas such as ammonia or N₂O which is usedin performing nitridation by heat treatment with a high temperature canbe used. The oxide film 104 contains a predetermined concentration ofthe rare gas which has been introduced into the second plasma treatmentchamber 202.

Plasma nitridation may be first performed to the semiconductor film 103in the second plasma treatment chamber 202 to form a nitride film.Further, plasma oxidation may be performed to the nitride film in thefirst plasma treatment chamber 201.

In the case of performing thermal oxidation to the semiconductor film103 in an oxygen atmosphere, an edge portion of the semiconductor film103, which is in contact with the base film 102, is oxidizedunintentionally. As a result, such a problem occurs that a thickness ofthe edge portion of the semiconductor film 103 becomes thinner than thatof other portions. This problem of thinning the film causes troubleparticularly when the semiconductor film 103 has a tapered shape.However, when plasma oxidation is performed, oxidation in an unintendedportion as described above is suppressed. The same can be said for thecase of plasma nitridation.

After forming the oxide film 104, a silicon nitride film or a siliconnitride film containing oxygen may be formed by a CVD method or the likeso as to form a gate insulating film together with the oxide film 104.Thus, oxidation of a gate electrode 105 and a wire pulled from the gateelectrode 105 later to be formed due to having contact with the oxidefilm 104 can be suppressed. Further, plasma nitridation with a lowelectron temperature and a high electron density may be performed to thesilicon nitride film or the silicon nitride film containing oxygen forthe purpose of densification.

Then, the gate electrode 105 and the wire pulled from the gate electrode105 are formed as shown in FIG. 1C. The gate electrode 105 and the wirepulled from the gate electrode 105 may be formed into a tapered shape,and a stack structure including two or more layers may be employed.Then, the semiconductor film 103 is doped with impurities impartingp-type conductivity or impurities imparting n-type conductivity and theimpurities are activated to form an impurity region 106 including asource region and a drain region. The impurity region 106 may include anLDD region as well as the source region and the drain region. Inaddition, the LDD region may be formed to overlap the gate electrode105.

A protective film 107 and an interlayer insulating film 108 are formedto cover the gate electrode 105 and the wire pulled from the gateelectrode 105, and contact holes exposing the source region and thedrain region are formed in the gate insulating film, the protective film107, and the interlayer insulating film 108. Then, wires 109 are formedto fill these contact holes and over the interlayer insulating film 108(see FIG. 1D). In forming the protective film 107, a silicon nitridefilm or a silicon nitride film containing oxygen is formed by a plasmaCVD method or the like. Plasma treatment with a low electron temperatureand a high electron density may be performed to the formed protectivefilm 107. Instead of the CVD method, plasma nitridation with a lowelectron temperature and a high electron density may be performed toform the protective film 107.

In one feature of this embodiment mode as described above, plasmatreatment with a low electron temperature and a high electron density isperformed to the semiconductor film 103 containing amorphous silicon soas to form a gate insulating film of a thin film transistor. In the caseof this embodiment mode, attention needs to be paid to that thesemiconductor film 103 becomes thin after performing the plasmatreatment. In the gate insulating film of this embodiment mode, plasmadamages and generation of cracks are suppressed, and heat treatment at ahigh temperature as in the thermal oxidation method is not required.Therefore, the gate insulating film can be formed at a temperature whichdoes not affect a glass substrate.

Embodiment Mode 2

In Embodiment Mode 2, a high-density plasma treatment apparatus as shownin FIGS. 2A and 2B is used, and a protective film is formed byperforming plasma treatment to a gate electrode of a thin filmtransistor.

Similarly to Embodiment Mode 1, a base insulating film 302 is formedover a glass substrate 301, and a semiconductor film 303 containingamorphous silicon is formed thereover with a predetermined pattern (seeFIG. 3A). Note that in this embodiment mode, when a predeterminedpattern is formed, a tapered shape is formed to have an angle θ in therange of 30° to 60°. Then, in later forming a gate insulating film by aCVD method or the like, superior step coverage can be obtained ascompared the case where an angle θ is in the range of 85° to 100°. Inaddition, in this embodiment mode also, a heat-resistant plasticsubstrate can be used, instead of using a glass substrate.

A gate insulating film 304 is formed over the semiconductor film 303(see FIG. 3B). The gate insulating film 304 is formed of a silicon oxidefilm containing nitrogen (silicon oxynitride film), a silicon nitridefilm containing oxygen, a silicon nitride film, or a silicon oxide filmby a plasma CVD method or the like. Further, by performing plasmanitridation or plasma oxidation, a nitride layer or an oxide layer canbe formed over a surface of the film which has been formed by a plasmaCVD method or the like. Alternatively, the gate insulating film 304 maybe formed by plasma treatment by the method described in Embodiment Mode1, instead of using a CVD method.

A gate electrode 305 shown in FIG. 3B and a wire pulled from the gateelectrode 305 are formed over the gate insulating film 304. A highmelting point metal film such as molybdenum, tungsten, or tantalumhaving a melting point of 2000° C. or more is formed by a sputteringmethod and formed into a wire shape in a photolithography step;accordingly, the gate electrode 305 is formed together with the wirepulled from the gate electrode 305. Instead of the sputtering method, amethod which does not require a photolithography step, for example, adroplet discharge (inkjet) method may be used. The gate electrode 305and the wire pulled from the gate electrode 305 may be formed into atapered shape, and a stack structure including two or more layers may beemployed.

Plasma nitridation is performed to the gate electrode 305 and the wirepulled from the gate electrode 305 in the second plasma treatmentchamber 202 of the high-density plasma treatment apparatus shown in FIG.2A so as to form a metal nitride (molybdenum nitride, tungsten nitride,tantalum nitride, or the like) over surfaces of the gate electrode 305and the wire pulled from the gate electrode 305. This metal nitride isto be a protective film 306 (see FIG. 3C). In a case where theprotective film 306 does not have insulating properties but hasconducting properties, the protective film 306 can be regarded as a partof the gate electrode 305. At this time, as apparently shown in FIG. 3C,a part of the gate insulating film 304, which is not covered by the gateelectrode 305, is also subjected to plasma treatment. The protectivefilm 306 contains a predetermined concentration of a rare gas which hasbeen introduced into the second plasma treatment chamber 202. The partof the gate insulating film 304, which is not covered by the gateelectrode 305 also contains the rare gas. Instead of the above-describedplasma nitridation, plasma oxidation described in Embodiment Mode 1 maybe performed as well, to form the protective film 306.

In this embodiment mode, microwaves having a frequency of 2.45 GHz areused in plasma nitridation, and nitrogen and argon are used as the gasintroduced into the second plasma treatment chamber 202. A temperatureof the heater provided under the substrate holder 211 is kept at 400° C.For example, a nitrogen flow is set in the range of 20 sccm to 2000sccm, and an argon flow is set in the range of 100 sccm to 10000 sccm. Apressure in the second plasma treatment chamber 202 is set at anappropriate value in the range of 5 Pa to 500 Pa. Hydrogen may befurther added into the nitrogen and argon, a gas made of a nitrogencompound such as ammonia may be substituted for the nitrogen, andanother rare gas may be substituted for the argon. In the case where aheat-resistant plastic substrate is used instead of the glass substrate301, a temperature of the heater provided under the substrate holder 211is kept at 250° C.

Since plasma over the gate electrode 305 and the wire pulled from thegate electrode 305 has an electron temperature of 1.5 eV or less and anelectron density of 1×10¹¹ cm⁻³ or more, plasma damages to theprotective film 306 obtained by plasma nitridation are suppressed.

The protective film 306 of this embodiment mode is formed to coverentire top and side surfaces of the gate electrode 305 and the wirepulled from the gate electrode 305. As a method of forming a protectivefilm over entire top and side surfaces of a gate electrode, a methodusing anodic oxidation can be given as an example. However, since notone thin film transistor but a plurality of thin film transistors isformed, it is necessary that all gate electrodes be connected such thateach has the same electric potential in anodic oxidation, and a step ofdividing into each gate electrode of a thin film transistor is requiredafter the anodic oxidation. On the other hand, in the case of formingthe protective film by plasma treatment, such a dividing step is notrequired. In addition, a material capable of being subjected to theanodic oxidation is limited to aluminum, tantalum, or the like.

Subsequently, the semiconductor film 303 is doped with impuritiesimparting p-type conductivity or impurities imparting n-typeconductivity and the impurities are activated, to form an impurityregion 307 including a source region and a drain region. This dopingstep may be performed before forming the protective film 306 and afterforming the gate electrode 305 and the wire pulled from the gateelectrode 305. Further, a second doping may be performed after formingthe protective film 306. The impurity region 307 may include an LDDregion in addition to the source region and the drain region. Inaddition, the LDD region may be formed to overlap the gate electrode305.

In this embodiment mode, since the protective film 306 is formed byplasma treatment, it is not necessary to form a film of, for example,silicon nitride or silicon nitride containing oxygen by a plasma CVDmethod or the like so as to form the protective film 306. Accordingly,after forming the protective film 306, as shown in FIG. 3D, aninterlayer insulating film 308 is formed to cover the gate electrode 305and the wire pulled from the gate electrode 305, contact holes exposingthe source region and the drain region are formed in the gate insulatingfilm 304 and the interlayer insulating film 308, and wires 309 areformed to fill these contact holes and over the interlayer insulatingfilm 308.

The wires 309 may have a stack structure including two or more layers.For example, three layers of a first titanium film, an aluminum film,and a second titanium film are sequentially formed by a sputteringmethod or the like. Further, plasma nitridation with a low electrontemperature and a high electron density described in this embodimentmode may be performed to the first titanium film to form a titaniumnitride layer over a surface of the first titanium film. It ispreferable to sequentially perform formation of the first titanium film,plasma nitridation, and formation of the aluminum film and the secondtitanium film without exposing to air. By forming films containing ametal as its main constituent such as chromium, molybdenum, or tungstenwhich has a higher melting point than that of aluminum, instead offorming the first and the second titanium films to interpose thealuminum film therebetween, a problem caused by low heat resistance ofaluminum can be solved as in the case of using the first and the secondtitanium films.

In this embodiment mode, before forming the gate insulating film 304,plasma oxidation or plasma nitridation may be performed to an edgeportion of the semiconductor film 303 with the high-density plasmatreatment apparatus as shown in FIGS. 2A and 2B. In the case where thesemiconductor film 303 has a tapered shape as in this embodiment mode,not only the impurity region 307 but also an edge portion of a channelformation region, which overlaps the gate electrode 305, of thesemiconductor film 303 actually has a tapered shape, although not shownin FIGS. 3C and 3D. Accordingly, due to this reason, a thin filmtransistor which uses the semiconductor film 303 sometimes showsdifferent characteristics from that in the case where the semiconductorfilm does not have a tapered shape. Such a thin film transistor iscalled a parasitic transistor, and the parasitic transistor can beprevented from being formed, by performing plasma oxidation or plasmanitridation to the edge portion (tapered portion) of the semiconductorfilm 303 and forming silicon oxide or silicon nitride thereover.

This embodiment mode can be carried out in combination with EmbodimentMode 1.

Embodiment Mode 3

In Embodiment Mode 3, plasma treatment is performed to an insulatingfilm (gate insulating film) formed by a plasma CVD or the like, with thehigh-density plasma treatment apparatus as shown in FIGS. 2A and 2B.Thus, a surface of this insulating film formed by a plasma CVD or thelike is modified to increase the quality of the gate insulating film.

Similarly to Embodiment Mode 2, a base insulating film 402 is formedover a glass substrate 401, and a semiconductor film 403 containingamorphous silicon is formed thereover with a predetermined pattern (seeFIG. 4A). In this embodiment mode also, a heat-resistant plasticsubstrate can be used instead of using the glass substrate.

An insulating film 404 is formed over the semiconductor film 403 by aplasma CVD method or the like. In this embodiment mode, a silicon oxidefilm containing nitrogen (silicon oxynitride film) is formed as theinsulating film 404. Instead of the silicon oxide film containingnitrogen, a silicon nitride film containing oxygen, a silicon oxidefilm, or a silicon nitride film may be formed by a CVD method or thelike.

Plasma nitridation is performed to the formed insulating film 404 in thesecond plasma treatment chamber 202 of the high-density plasma treatmentapparatus shown in FIG. 2A. The insulating film 404 contains apredetermined concentration of a rare gas which has been introduced intothe second plasma treatment chamber 202. The insulating film 404subjected to plasma nitridation is used as a gate insulating film (seeFIG. 4B).

In this embodiment mode, microwaves having a frequency of 2.45 GHz areused in plasma nitridation, and nitrogen and argon are used as the gasintroduced into the second plasma treatment chamber 202. A temperatureof the heater provided under the substrate holder 211 is kept at 400° C.The nitrogen and argon flows are set in the range described inEmbodiment Mode 2. Hydrogen may be further added into the nitrogen andargon, a gas made of a nitrogen compound such as ammonia may besubstituted for the nitrogen, and another rare gas may be substitutedfor the argon. In the case where a heat-resistant plastic substrate isused instead of the glass substrate 401, a temperature of the heaterprovided under the substrate holder 211 is kept at 250° C., for example.Plasma over the insulating film 404 has an electron temperature of 1.5eV or less and an electron density of 1×10¹¹ cm⁻³ or more.

Instead of plasma nitridation, plasma oxidation may be performed in thefirst plasma treatment chamber 201 of the high-density plasma treatmentapparatus shown in FIG. 2A.

There is a case where dust is attached to a film which is formed by aCVD method or a sputtering method. Although various shapes of this dustcan be considered, a state where granular dust 501 formed from aninorganic substance is attached to a surface of the insulating film 404is shown in FIG. 5A. A case where plasma nitridation or plasma oxidationis performed to the insulating film 404 on which the dust 501 isattached is considered in accordance with this embodiment mode. Notethat the above-mentioned dust is also called a particle, and a filmformed by a CVD method, a sputtering method, or the like is required tohave as little particles as possible.

By the plasma oxidation or plasma nitridation, oxidation or nitridationproceeds to a portion under the dust 501, as well as a portion on whicha dust does not exist (see FIG. 5B). A thickness of the insulating film404 increases by the plasma oxidation or plasma nitridation, and athickness of the portion under the dust 501 similarly increases as well.In addition, at least a surface portion 502 of the dust 501 is oxidizedor nitrided. As a result, a volume of the dust 501 is increased. Notethat when the insulating film 404 and the dust 501 are formed of anitride and plasma nitridation is performed thereto, or when theinsulating film 404 and the dust 501 are formed of an oxide and plasmaoxidation is performed thereto, a volume of the dust 501 does notincrease and a surface of the insulating film 404 is not nitrided oroxidized.

When the thickness of the insulating film 404 and the volume of the dust501 are increased, as shown in FIG. 5B, a state in which the dust 501can be easily removed from the surface of the oxidized or nitridedinsulating film 404 by a simple cleaning method such as brush cleaningor megasonic cleaning can be obtained. Thus, even a dust with a size ofseveral nanometers can become easy to be removed by plasma oxidation orplasma nitridation. This can be said not only in this embodiment mode,and the same can be said for other embodiment modes in the case whereplasma treatment is performed to a gate electrode or a semiconductorfilm to which dust (particle) is attached.

The above explanation is for the case where the dust (particle) isformed from an inorganic substance; however, in the case where the dustis formed from an organic substance, ashing is performed by plasmaoxidation and the dust can be removed without separately performingcleaning.

After performing the plasma treatment to the insulating film 404, a gateelectrode 405 and a wire pulled from the gate electrode 405 are formedas shown in FIG. 4C. The gate electrode 405 and the wire pulled from thegate electrode 405 may have a tapered shape, and a stack structureincluding two or more layers may be employed. Subsequently, thesemiconductor film 403 is doped with impurities imparting p-typeconductivity or impurities imparting n-type conductivity and theimpurities are activated, to form an impurity region 406 including asource region and a drain region. The impurity region 406 may include anLDD region in addition to the source region and the drain region. Inaddition, the LDD region may be formed to overlap the gate electrode405.

A protective film 407 and an interlayer insulating film 408 are formedto cover the gate electrode 405 and the wire pulled from the gateelectrode 405, contact holes exposing the source region and the drainregion are formed in the insulating film 404, the protective film 407,and the interlayer insulating film 408. Then, wires 409 are formed tofill these contact holes and over the interlayer insulating film 408(see FIG. 4D). In forming the protective film 407, a silicon nitridefilm or a silicon nitride film containing oxygen is formed by a plasmaCVD method or the like. Plasma treatment with a low electron temperatureand a high electron density can be performed to the formed protectivefilm 407. As the protective film 407, a silicon oxide film is formed bya plasma CVD method or the like, and plasma nitridation with a lowelectron temperature and a high electron density may be performedthereto. The protective film 407 may be formed by plasma nitridationwith a low electron temperature and a high electron density as inEmbodiment Mode 2, instead of using a CVD method.

In this embodiment mode, when the semiconductor film 403 has a taperedshape as shown in FIG. 4A, an edge portion (tapered portion) of thesemiconductor film 403 may be subjected to plasma oxidation or plasmanitridation before forming the gate insulating film 404.

This embodiment mode can be carried out in combination with either orboth of Embodiment Mode 1 and Embodiment Mode 2.

Embodiment Mode 4

In Embodiment Mode 4, an example of performing plasma nitridation orplasma oxidation with the high-density plasma treatment apparatus asshown in FIGS. 2A and 2B in a manufacturing process of a bottom-gatethin film transistor will be described.

As shown in FIG. 6A, a gate electrode 602 and a wire pulled from thegate electrode 602 are formed over a glass substrate 601. In addition, ahigh melting point metal film such as molybdenum, tungsten, or tantalumhaving a melting point of 2000° C. or more is formed by a sputteringmethod and formed into a wire shape in a photolithography step;accordingly, the gate electrode 602 is formed together with the wirepulled from the gate electrode 602. Instead of the sputtering method, amethod which does not require a photolithography step, for example, adroplet discharge (inkjet) method may be used. A heat-resistant plasticsubstrate may be used instead of the glass substrate. In this embodimentmode, the gate electrode 602 and the wire pulled from the gate electrode602 may be formed into a tapered shape as shown in FIG. 6A; however,those are not necessarily formed into a tapered shape.

In addition, the gate electrode 602 and the wire pulled from the gateelectrode 602 may be formed in such a manner as described in EmbodimentMode 2, and a stack structure including two or more layers may beemployed.

Plasma oxidation is performed to the gate electrode 602 and the wirepulled from the gate electrode 602 in the first plasma treatment chamber201 of the high-density plasma treatment apparatus shown in FIG. 2A soas to form a metal oxide (molybdenum oxide, tungsten oxide, tantalumoxide, or the like) over surfaces of the gate electrode 602 and the wirepulled from the gate electrode 602. This metal oxide is shown as a firstprotective film 603 in FIG. 6B. At the same time, as is apparently shownin FIG. 6B, the glass substrate 601 is also subjected to plasmatreatment. The oxide film and the glass substrate 601 contain apredetermined concentration of a rare gas which has been introduced intothe first plasma treatment chamber 201.

In plasma oxidation of this embodiment mode, plasma is generated usingmicrowaves having a frequency of 2.45 GHz, and oxygen, hydrogen, andargon are introduced into the first plasma treatment chamber 201 with aflow ratio of O₂:H₂:Ar=1:1:100, for example. The flows of oxygen,hydrogen, and argon are set in the range described in Embodiment Mode 1.Plasma oxidation can be performed without introducing hydrogen,similarly to Embodiment Mode 1. Instead of the argon, another rare gasmay be introduced. A pressure in the first plasma treatment chamber 201is set at an appropriate value in the range of 5 Pa to 500 Pa. The glasssubstrate 601 is provided on the substrate holder 211 of the firstplasma treatment chamber 201, and a temperature of the heater providedunder the substrate holder 211 is kept at 400° C. Then, plasma oxidationis performed to the gate electrode 602 and the wire pulled from the gateelectrode 602 over the glass substrate 601. As a result, the firstprotective film 603 shown in FIG. 6B is formed. In the case where aheat-resistant plastic substrate is used instead of the glass substrate601, a temperature of the heater provided under the substrate holder 211is kept at, for example, 250° C.

Since plasma over the gate electrode 602 and the wire pulled from thegate electrode 602 has an electron temperature of 1.5 eV or less and anelectron density of 1×10¹¹ cm⁻³ or more, plasma damages to the oxidefilm obtained by plasma oxidation are suppressed.

The first protective film 603 of this embodiment mode is formed to coverentire top and side surfaces of the gate electrode 602 and the wirepulled from the gate electrode 602. As a method of forming a protectivefilm over entire top and side surfaces of a gate electrode, a methodusing anodic oxidation is known. However, since not one thin filmtransistor but a plurality of thin film transistors is formed, it isnecessary that all gate electrodes be connected such that each has thesame electric potential in anodic oxidation, and a step of dividing intoeach gate electrode of a thin film transistor is required after theanodic oxidation. On the other hand, in the case of forming the oxidefilm by plasma treatment, such a dividing step is not required.

Instead of plasma oxidation, plasma nitridation may be performed by themethod described in Embodiment Mode 2 to form the first protective film603. In that case, a metal nitride (molybdenum nitride, tungstennitride, tantalum nitride, or the like) is formed. Plasma nitridationmay be continuously performed after plasma oxidation, or plasmaoxidation may be continuously performed after plasma nitridation aswell.

When the first protective film 603 is an insulating film of molybdenumoxide, tungsten oxide, tantalum oxide, or the like, the first protectivefilm 603 can be a part of the gate insulating film.

An insulating film 604 is formed over the first protective film 603 andthe glass substrate 601 by a plasma CVD method or the like (see FIG.6C). In this embodiment mode, a silicon oxide film containing nitrogen(silicon oxynitride film) is formed as the insulating film 604. Insteadof the silicon oxide film containing nitrogen, a silicon nitride film, asilicon nitride film containing oxygen, or a silicon oxide film may beformed by a CVD method or the like.

Plasma nitridation is performed to the insulating film 604 in the secondplasma treatment chamber 202 of the high-density plasma treatmentapparatus shown in FIG. 2A. The insulating film 604 subjected to plasmanitridation is used as a gate insulating film.

In plasma nitridation of this embodiment mode, microwaves having afrequency of 2.45 GHz are used, nitrogen and argon are used as a gasintroduced into the second plasma treatment chamber 202, and atemperature of the heater provided under the substrate holder 211 iskept at 400° C. The flows of nitrogen and argon are set in the rangedescribed in Embodiment Mode 2. Hydrogen may be further added to thenitrogen and argon, ammonia may be substituted for the nitrogen, andanother rare gas may be substituted for the argon. In the case where aheat-resistant plastic substrate is used instead of the glass substrate601, a temperature of the heater provided under the substrate holder 211is kept at 250° C. The plasma over the insulating film 604 has anelectron temperature of 1.5 eV or less and an electron density of 1×10¹¹cm⁻³ or more.

Instead of plasma nitridation, plasma oxidation may be performed in thefirst plasma treatment chamber 201 of the high-density plasma treatmentapparatus shown in FIG. 2A.

Then, as shown in FIG. 6D for example, a first semiconductor film 605containing amorphous silicon, a second protective film 606, a secondsemiconductor film 607 doped with impurities imparting n-typeconductivity (such as phosphorus), and a wire 608 are formed with apredetermined shape by a known method. The impurities such as phosphoruscontained in the second semiconductor film 607 are activated ifnecessary. The second protective film 606 is usually called a channelprotective film.

The bottom-gate thin film transistor manufactured in this embodimentmode is not limited to the structure shown in FIG. 6D. A bottom-gatethin film transistor having another structure such as a channel-etchedthin film transistor without a channel protective film may also be used.

Described above is the example of performing plasma treatment with a lowelectron temperature and a high electron density to both the gateelectrode 602 and the wire pulled from the gate electrode 602, and theinsulating film 604. However, the plasma treatment with a low electrontemperature and a high electron density may be performed to either oneof them. In addition, when the first protective film 603 sufficientlyfunctions as a gate insulating film, the first protective film 603 maybe used as a gate insulating film without providing the insulating film604.

Embodiment Mode 5

Embodiment Mode 5 describes an example of performing plasma treatment toa protective film after forming the protective film. The protective filmcorresponds to the protective film 107 shown in Embodiment Mode 1 andFIG. 1D or the protective film 407 shown in Embodiment Mode 3 and FIG.4D.

A process up to forming the protective film may follow Embodiment Mode 1or Embodiment Mode 3. Alternatively, the plasma treatment to theinsulating film 404 carried out in Embodiment Mode 3 may be skipped anda process up to forming the protective film 404 may be performed. FIG.7A shows a state, in which the protective film 407 is formed byfollowing Embodiment Mode 3, in other words, through plasma treatment tothe insulating film 404. In this embodiment mode, a silicon nitridefilm, a silicon nitride film containing oxygen, or a silicon oxide filmformed by a plasma CVD method or the like is used as the protective film407. The reference numerals 401 to 407 shown in FIG. 7A commonly denotethe same components in Embodiment Mode 3.

Next, as shown in FIG. 7B, after forming the protective film 407, plasmatreatment is performed in the second plasma treatment chamber 202 of thehigh-density plasma treatment apparatus shown in FIG. 2A. In the plasmatreatment, microwaves having a frequency of 2.45 GHz are used, andhydrogen and a rare gas are used as the gas introduced into the secondplasma treatment chamber 202. A temperature of the heater provided underthe substrate holder 211 is kept at 350° C. or more and 450° C. or less.As the rare gas, argon is used in this embodiment mode. For example, ahydrogen flow is set in the range of 20 sccm to 2000 sccm, and an argonflow is set in the range of 100 sccm to 10000 sccm. The plasma over theprotective film 407 has an electron temperature of 1.5 eV or less and anelectron density of 1×10¹¹ cm⁻³ or more. Reference character, “H*” shownin FIG. 7B means a hydrogen radical.

Since hydrogen is used as the introduction gas as described above, theprotective film 407 after the plasma treatment contains hydrogen. Sincethe glass substrate 401 is heated in the plasma treatment, hydrogen inthe protective film 407 diffuses into the semiconductor film 403containing amorphous silicon through the insulating film 404 tohydrogenate the semiconductor film 403. Hydrogen is also diffused into achannel formation region below the gate electrode 405 as shown in FIG.7B. After the plasma treatment, the glass substrate 401 may be heatedfor a predetermine time with a temperature of 350° C. or more and 400°C. or less in an atmosphere containing hydrogen so as to furtherhydrogenate the semiconductor film 403.

In addition, as the gas introduced into the second plasma treatmentchamber 202, ammonia (NH₃) can be added to the hydrogen and argon, orammonia can be substituted for the hydrogen. In this case, hydrogen isintroduced from a surface of the protective film 407, the semiconductorfilm 403 can be hydrogenated, and plasma nitridation can also beperformed to the protective film 407. When the protective film 407 is asilicon nitride film containing oxygen, at least a surface of theprotective film 407 is nitrided. When the protective film 407 is asilicon oxide film, at least a surface of the protective film 407 isnitrided to form silicon oxynitride. When the protective film 407 is asilicon nitride film, densification thereof can be achieved.

In addition, when hydrogen is contained in the gas introduced to thefirst plasma treatment chamber 201 or the second plasma treatmentchamber 202 in performing plasma treatment with a low electrontemperature and a high plasma density to the insulating film 404,hydrogen is added to the insulating film 404. Then, the glass substrate401 is heated at a temperature of the heater provided under thesubstrate holder 211 of 350° C. or more and 450° C. or lower to diffusethe added hydrogen into the semiconductor film 403, and thesemiconductor film 403 can be hydrogenated. In addition, nitridation andoxidation may be prevented from being performed in plasma treatment byusing only hydrogen and a rare gas as the introduction gas.

In the case where the semiconductor film 403 is doped and activatedafter hydrogenating the semiconductor film 403, when the activation isperformed at a temperature of 500° C. or more, hydrogen is removed fromthe semiconductor film 403. Accordingly, an order should be changed asfollows: the insulating film 404 and the gate electrode 405 are formed,doping and activation at a temperature of 500° C. or more are performedto the semiconductor film 403, and then the semiconductor film 403 ishydrogenated by the plasma treatment as shown in FIG. 7C. Referencecharacter, “H*” shown in FIG. 7C means a hydrogen radical. After that,the glass substrate 401 may be heated at a temperature of 350° C. ormore and 400° C. or less for a predetermined time in an atmospherecontaining hydrogen so as to further hydrogenate the semiconductor film403.

Hydrogenation in this embodiment mode can be carried out in combinationwith other embodiment modes.

Embodiment Mode 6

In Embodiment Mode 6, an example of performing plasma nitridation to aglass substrate with the high-density plasma treatment apparatus asshown in FIGS. 2A and 2B will be described.

The glass substrate used in Embodiment Modes 1 to 5 is typically anon-alkali glass. The non-alkali glass contains silicon oxide as itsmain component and contains boron oxide, aluminum oxide, and an oxide ofan alkaline earth metal. By performing plasma nitridation to such anon-alkali glass, a nitride layer containing silicon nitride or siliconnitride containing oxygen as its main component can be formed over asurface thereof.

Accordingly, when performing plasma nitridation to the glass substratewith a low electron temperature and a high electron density inEmbodiment Mode 1, 2, 3, or 5, a silicon nitride film or a siliconnitride film containing oxygen is not necessarily formed by a CVD methodor the like as a base insulating film. In addition, plasma damages canbe suppressed more and a denser nitride film can be formed than in thecase of forming the silicon nitride film or the silicon nitride filmcontaining oxygen by a CVD method.

Embodiment Mode 7

In Embodiment Mode 7, an example of employing a multigate structure as astructure of a thin film transistor will be described. The multigatestructure is a structure in which two or more thin film transistorshaving standard structures (single-gate structure) shown in FIG. 1D orthe like are connected in series and in which gate electrodes of therespective thin film transistors are connected to each other. It isknown that off current can be reduced by employing the multigatestructure, compared with the case of the single-gate structure.

The plasma treatment described in Embodiment Modes 1 to 6 can be appliedto a manufacturing process of a thin film transistor having a multigatestructure. The similar effect to that in the case of the thin filmtransistor having a single-gate structure can be obtained, by performingplasma oxidation or plasma nitridation with a low electron temperatureand a high electron density in manufacturing the thin film transistorhaving a multigate structure.

Embodiment Mode 8

Oxidation characteristics in performing plasma oxidation to a treatmentsubject with the above-described high-density plasma treatment apparatuswill be described. Specifically, changes in oxidative rates due to adifference of gases used in the plasma oxidation will be described.

First, a silicon oxynitride film (SiO_(x)N_(y) film, note that x>y) isformed with a thickness of about 100 nm as a base insulating film over aglass substrate by a CVD method, and an amorphous silicon film is formedwith a thickness of about 66 nm over the base insulating film by a CVDmethod. Next, heat treatment is performed to remove hydrogen containedin the amorphous silicon film, and then, the amorphous silicon film iscrystallized by laser light irradiation to form a crystalline siliconfilm. Subsequently, plasma oxidation is performed to the crystallinesilicon film with the high-density plasma treatment apparatus. In plasmaoxidation, the glass substrate is set on a substrate holder, and atemperature of a heater provided under the substrate holder is set at400° C.

In this embodiment mode, plasma oxidation is performed with flows ofargon and oxygen set at 500 sccm and 5 sccm respectively (Condition 1),or with flows of argon, oxygen, and hydrogen set at 500 sccm, 5 sccm,and 5 sccm respectively (Condition 2). In Conditions 1 and 2, pressureis set at 133.33 Pa. The only difference between Conditions 1 and 2 iswhether hydrogen is introduced or not.

Oxidative rates of amorphous silicon films of Conditions 1 and 2 areshown in FIG. 11. Note that the horizontal axis shows the treating time(sec), and the vertical axis shows the average film thickness (nm) inFIG. 11. The treating time means time the plasma oxidation is performedto the amorphous silicon film. The average film thickness means a resultof an average value which is obtained by measuring film thicknesses of25 portions of an oxide film formed by oxidizing the amorphous siliconfilm by plasma oxidation.

In Conditions 1 and 2, as the treating time by the high-density plasmatreatment apparatus is increased, oxidation of the amorphous siliconfilm proceeds, and an average film thickness of the oxide film formed onthe amorphous silicon film is increased. In addition, comparing with thecase where plasma oxidation is performed with Condition 1 which is thecase where argon and oxygen are introduced, it is found that an averagefilm thickness of the oxide film formed on the amorphous silicon film isthicker when plasma oxidation is performed with Condition 2 which is thecase where hydrogen is added in Condition 1. In other words, it is foundthat by performing plasma oxidation with the condition of addinghydrogen, a treating time for forming an oxide film having apredetermined thickness can be reduced, and a thicker oxide film can beformed in the same treating time, compared with the condition withoutadding hydrogen.

Embodiment 1

As an example of using the thin film transistor manufactured inEmbodiment Modes 1 to 7 in an active matrix display device, an EL(electroluminescence) display device including a light emitting elementin a pixel portion is described.

FIG. 8A is a top view showing an example of the active matrix displaydevice, and FIG. 8B shows a cross-section of the EL display device,taken along line g-h of FIG. 8A.

As shown in FIG. 8A, the display device shown in this embodimentincludes a pixel portion 704 provided over a substrate 701. In addition,an opposite substrate 706 is provided to face the substrate 701 so as tointerpose the pixel portion 704 therebetween. The pixel portion 704 isprovided with a thin film transistor having any structure described inEmbodiment Modes 1 to 7 over the substrate 701. The substrate 701 andthe opposite substrate 706 are attached with a sealing material 705. Inaddition, a driver circuit is provided outside the substrate 701 via anFPC (Flexible Printed Circuit) 707 in which a wire is formed of copperfoil or the like.

The pixel portion 704 is formed of a plurality of pixels each of whichincludes a light emitting element 716 and a thin film transistor 711 fordriving the light emitting element 716 shown in FIG. 8B. As the thinfilm transistor 711, the thin film transistor manufactured through anyprocess shown in Embodiment Modes 1 to 7 can be employed.

In addition, in this embodiment, a first electrode 713 is provided toconnect to a wire 712 which is connected to a source or drain region ofthe thin film transistor 711, and an insulating film 709 is formed tocover an edge portion of the first electrode 713. The insulating film709 functions as a partition wall between the plurality of pixels.

The insulating film 709 is formed by using a positive photosensitiveacrylic resin film. In addition, in order to obtain favorable coverage,the insulating film 709 is provided so as to form a curved surfacehaving a curvature radius at an upper portion or a lower portion of theinsulating film 709. For example, when a positive photosensitive acrylicis used as a material of the insulating film 709, it is preferable thatonly the upper portion of the insulating film 709 have a curved surfacehaving a curvature radius (0.2 μm to 3 μm). As the insulating film 709,either of a negative type which is photosensitive and becomes insolublein an etchant by light or a positive type which becomes soluble in anetchant by light can be used. In addition, as the insulating film 709, asingle layer or a stack structure of an organic material such as epoxy,polyimide, polyamide, polyvinylphenol, or benzocyclobutene or asiloxane-based resin can be used. In addition, plasma treatment can beperformed to the insulating film 709 to oxidize or nitride theinsulating film 709; accordingly, a surface of the insulating film 709is modified and a dense film can be obtained. By modifying the surfaceof the insulating film 709, strength of the insulating film 709 isimproved, and physical damages can be reduced, such as generation ofcracks in forming an opening portion or the like or film reduction inetching. In addition, by modifying the surface of the insulating film709, interface properties such as attachment to a light emitting layer714 provided over the insulating film 709 can be improved.

In the EL display device shown in FIGS. 8A and 8B, the light emittinglayer 714 is formed over the first electrode 713, and a second electrode715 is formed over the light emitting layer 714. A light emittingelement 716 is formed of a stack structure of the first electrode 713,the light emitting layer 714, and the second electrode 715.

One of the first electrode 713 and the second electrode 715 is used asan anode, and the other is used as a cathode. In the case of being usedas the anode, a material with a high work function is preferably used.For example, not only a single-layer film such as an ITO film, an indiumtin oxide film containing silicon, a light-transmitting conductive filmformed with a target in which zinc oxide (ZnO) is mixed with indiumoxide by a sputtering method, zinc oxide (ZnO), a titanium nitride film,a chromium film, a tungsten film, a Zn film, or a Pt film, but also astack of a titanium nitride film and a film containing aluminum as itsmain component, a three-layered structure of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm can be used. Note that when employing a stack structure, resistanceof a wire becomes low, favorable ohmic contact can be obtained, andfurther the stack structure can function as an anode. In the case ofbeing used as the cathode, a material with a low work function (Al, Ag,Li, Ca, or an alloy thereof such as MgAg, MgIn, AlLi, CaF₂, or calciumnitride) is preferably used. Note that in a case of making the electrodeused as the cathode have light transmitting properties, a stack of ametal thin film a thickness of which is thinned and a light-transmittingconductive film is preferably used as the electrode. As thelight-transmitting conductive film, for example, ITO, indium tin oxidecontaining silicon, a light-transmitting conductive film formed with atarget in which zinc oxide (ZnO) is mixed with indium oxide by asputtering method, or zinc oxide (ZnO) can be used. Here, an ITO havinglight transmitting properties is used as the first electrode 713, and astructure in which light is extracted from a side of the substrate 701is employed. Note that a structure in which light is extracted from aside of the opposite substrate 706 may be used by using a lighttransmitting material for the second electrode 715. Alternatively, astructure in which light is extracted from both sides of the substrate701 and the opposite substrate 706 (dual emission) can be used as wellby forming the first electrode 713 and the second electrode 715 with alight transmitting material.

In addition, the light emitting layer 714 can be formed of a singlelayer or a stack structure of a low molecular material, a middlemolecular material (including oligomer and dendrimer), or a highmolecular material by a known method such as a vapor-deposition methodusing a vapor-deposition mask, an inkjet method, or a spin coatingmethod.

In addition, a structure is used, in which the light emitting element716 of the invention is provided in a space 708 which is surrounded bythe substrate 701, the opposite substrate 706, and the sealing material705 by attaching the opposite substrate 706 and the substrate 701 withthe sealing material 705. Note that a structure of filling the space 708with the sealing material 705 can be also employed in addition to thecase of filling the space 708 with an inert gas (such as nitrogen orargon).

Note that epoxy resin is preferably used as the sealing material 705. Asa material used for the opposite substrate 706, a plastic substrate madefrom FRP (Fiberglass-Reinforced Plastics), PVF (polyvinyl fluoride),mylar, polyester, acryric, or the like can be used as well as a glasssubstrate or a quartz substrate.

Embodiment 2

As an example of using the thin film transistor manufactured inEmbodiment Modes 1 to 7 in an active matrix display device, a liquidcrystal display device in which a liquid crystal is used in a pixelportion is described.

FIG. 9 shows an example of a liquid crystal display device, and is across-section taken along line g-h shown in FIG. 8A. A liquid crystal822 is provided between an orientation film 821 formed to cover a wire812 and a first electrode 813 and an orientation film 823 formed belowan opposite substrate 706. In addition, a second electrode 824 isprovided on the opposite substrate 706, and voltage applied to theliquid crystal 822 which is provided between the first electrode 813 andthe second electrode 824 is controlled, so as to control transmission oflight and display an image.

In addition, a spherical spacer 825 is provided in the liquid crystal822 to control gap (cell gap) between the substrate 701 and the oppositesubstrate 706. The spacer 825 is not limited to a spherical shape, and acolumnar spacer may be provided. The substrate 701 and the oppositesubstrate 706 are attached with the sealing material 705. A thin filmtransistor manufactured through any process shown in Embodiment Modes 1to 7 can be applied to the thin film transistor 811.

Embodiment 3

In Embodiment 3, usage modes of the active matrix display devicedescribed in Embodiments 1 and 2 will be described with reference todrawings.

An example of an electronic device in which the above-mentioned activematrix display device is incorporated is described. For example, a videocamera, a digital camera, a goggle type display (head-mounted display),a television set, a navigation system, a sound-reproducing device suchas a car audio, a laptop computer, a game machine, a portableinformation terminal (such as a mobile computer, a cellular phone, aportable game machine, or an electronic book), and a portable imagereproducing device equipped with a recording medium can be given. Theinvention disclosed in this specification can be applied to a displayportion of these electronic devices.

FIG. 10A shows an example of the television set and includes a chassis1001, a display portion 1002, a speaker 1003, an operating portion 1004,a video input terminal 1005, and the like. By applying the thin filmtransistor manufactured in accordance with the invention disclosed inthis specification to the display portion 1002, the television set canbe manufactured. Since the thin film transistor manufactured inaccordance with the invention disclosed in this specification is used ina pixel of the display portion 1002, there are few pixel defects. If adefect exists, it cannot be recognized by the human eyes. Accordingly, abright and clear image can be displayed in the display portion 1002without a display fault.

An example of the digital camera is shown in FIGS. 10B and 10C. FIG. 10Bis a front view of the digital camera, and reference numeral 1011denotes a release button; 1012, a main switch; 1013, a viewfinder; 1014,a stroboscope; 1015, a lens; and 1016, a chassis. FIG. 10C is a backview of the digital camera, and reference numeral 1017 denotes aviewfinder eyepiece window; 1018, a display portion; 1019, an operatingbutton; and 1020, an operating button.

By applying the thin film transistor manufactured in accordance with theinvention disclosed in this specification to the display portion 1018,the digital camera can be manufactured. Since the thin film transistormanufactured in accordance with the invention disclosed in thisspecification is used in a pixel of the display portion 1018, there arefew pixel defects. If a defect exists, it cannot be recognized by thehuman eyes. Accordingly, a bright and clear image can be displayed inthe display portion 1018 without a display fault.

It is obvious that the invention disclosed in this specification is notlimited to the television set and the digital camera and can be appliedto an active matrix display device which is incorporated in anelectronic device including a display portion.

This application is based on Japanese Patent Application serial no.2005-133713 filed in Japan Patent Office on Apr. 28, 2005, the entirecontents of which are hereby incorporated by reference.

1. A manufacturing method of a thin film transistor, comprising thesteps of: forming a base insulating film over a glass substrate; forminga pattern of a semiconductor film containing silicon over the baseinsulating film, wherein the pattern of the semiconductor film has atapered shape at an edge portion of the pattern; forming a firstinsulating film by performing plasma oxidation using high electrondensity plasma with a frequency of 2.45 GHz to the pattern of thesemiconductor film with a condition where a temperature of the glasssubstrate is set at a temperature 100° C. or more lower than a strainpoint of the glass substrate so that a thickness of a top portion of thefirst insulating film, a thickness of a side portion of the firstinsulating film and a corner portion of the first insulating film aresame; forming a second insulating film containing nitrogen over thefirst insulating film by a CVD method; performing plasma nitridation tothe second insulating film; forming a gate electrode over the secondinsulating film, wherein oxygen gas, hydrogen gas and an inert gas areprovided to perform plasma oxidation, and wherein the base insulatingfilm is formed with plasma nitridation of the glass substrate so as toform a silicon nitride or silicon nitride containing oxygen.
 2. Themanufacturing method of a thin film transistor according to claim 1,wherein the plasma oxidation is performed with a plasma which has anelectron temperature of 0.5 eV or more and 1.5 eV or less and anelectron density of 1×10¹¹ cm⁻³ or more and 1×10¹³ cm⁻³ or less.
 3. Themanufacturing method of a thin film transistor according to claim 1,further comprising: forming a protective film over the gate electrode,wherein the protective film is in contact with the base insulating film.4. The manufacturing method of a thin film transistor according to claim1, wherein the first insulating film includes argon.
 5. Themanufacturing method of a thin film transistor according to claim 4,wherein concentration of the argon in the first insulating film is1×10¹⁵ atoms/cm³ or more and 1×10¹⁶ atoms/cm³ or less.
 6. Themanufacturing method of a thin film transistor according to claim 1,wherein the second insulating film containing nitrogen is a siliconnitride film.
 7. The manufacturing method of a thin film transistoraccording to claim 1, wherein hydrogen gas volume ratio to oxygen gasvolume is 1.5 or less.
 8. The manufacturing method of a thin filmtransistor according to claim 1, wherein the base insulating filmcomprises a silicon oxide containing nitrogen and a silicon nitridecontaining oxygen.
 9. The manufacturing method of a thin film transistoraccording to claim 1, wherein plasma nitridation of the glass substrateis performed by using high electron density plasma with a frequency of2.45 GHz.